In conventional intravenous bag systems when the bags are totally depleted (i.e. “run dry”), the previously placed air then is allowed to leave the bag. This usually then fills the drip chamber and the plastic tubing of the intravenous line. This can be problematic in many settings.
Once the tubing has air inside of it, a new bag must be hung, the tubing transferred to it, and the line must be re-primed. Re-priming involves placing a syringe and needle into a port on the intravenous tubing and withdrawing the air from the tubing. This takes time that can be problematic when the patient needs intravenous medications or acute fluid administration for a sudden change in their vital signs (i.e. blood pressure, heart rate, etc.). In fact, these intravenous bags usually are not noticed “running dry” during emergencies because everyone's attention is usually focused on other things. During true emergent traumas, a patient may be getting intravenous solution more rapidly then with the standard gravity drip. Often times the intravenous bags are placed in pressurized bags or machines that literally squeeze the bag forcing the solution into the patient via the intravenous tubing. Unfortunately, when all the fluid is pressurized out of the bag, the entrapped air is next forced through the intravenous tubing, potentially into the patient.
Air in the intravenous tubing is potentially disastrous because enough air may cause a “vapor-lock” phenomena whereby the right ventricle of the heart fills with air. Normal contractions are ineffective to push blood through the pulmonary vasculature where it is oxygenated and delivered to the left ventricle to be pushed out and circulated into the body. In other words, vapor-lock is a sudden cardiovascular collapse where no more blood can be circulated. An adult would need a high amount of air but a pediatric patient with a smaller heart would require much less air to cause this fatal scenario. Another potential problem is that air may not collect in the right ventricle, but may get pushed into the pulmonary vasculature. The name for this potentially lethal event is called pulmonary embolism. Air may get stuck in the pulmonary capillaries. This causes an increased resistance to the normal forward flow to the left atrium of blood. This increased resistance may cause the right side of the heart to fail, also since blood is not being circulated, the oxygen content falls, and since it is not getting to the left side of the heart, the output from the heart into the body drops to critical levels.
The above two scenarios are certainly possible but require large amounts of air. The most likely scenario for air entering into the vasculature and causing a devastating complication is via a probe patent or even an open Foramen Ovale. Bile Foramen Ovale is a unique fetal adaptation the human heart has while the fetus in the uterus. Blood is shunted away from the lungs (since the fetus is not breathing) and into the main vasculature. One way this blood is shunted past the lungs in through a hole in the septum between the right and left atrium of the heart. This hole is called the Foramen Ovale. Normally this hole closes right alter birth as the human heart now directs blood into the lungs than past them.
Unfortunately, in up to 15% of adults and a much higher corresponding level of neonates and children, this percentage is even higher. Probe patent means that a probe can be pushed through the Foramen Ovale which is only partially closed, or in other situations, it might be completely open. If a small air bubble makes it to the right atrium, the normal mechanism of passing into the right ventricle and then getting lodged into the palmary vasculature is bypassed. Instead, this air bubble may pass through the Foramen Ovale into the left atrium (bypassing the lungs), entering into the left ventricle, and then squeezed out into the body. If this air bubble goes to the brain, a devastating stroke may ensue. Central lines, which are long catheters intravenously placed into large veins and threaded close to the heart are more likely to cause this situation, however, even a small peripheral intravenous line can still elicit this situation especially in the setting of a small pediatric patient.
The above reasons are why medical practitioners are so adamant on not allowing any air to pass into the patient. Unfortunately, with the current intravenous bags that are in use today, this is a constant threat.